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Rapid global warming of about 6 degrees Celsius some 59 million years ago triggered a drastic change in ocean
circulation that lasted thousands of years. Carbon dioxide began to build up in the
atmosphere, slowly warming the sea
surface in the high latitudes. The warming threw a wrench into ocean
circulation and chemistry, which caused extinctions of deep-sea species
and land mammals.

A study by Nunes and Norris published in Nature today reports that deep sea sediments indicate deep sea currents had
previously formed in the Southern Ocean and South Atlantic Ocean
and flowed north to the Pacific Ocean. But less than 5000 years
later, the currents reversed direction and this reverse flow lasted at least 40,000 years. Modern atmospheric CO2
levels are approaching those seen 55 million years ago and could have a similarly profound impact on ocean circulation.Nature report

This is the North Atlantic conveyor belt slowing/reversal, an abrupt climate change scenario that in which a deep freeze of Europe and North America occurs due to global warming. Such regional cooling would eventually be overcome by gradual global warming, but North America and Europe could experience bitter winters such as those that occurred during the "little ice age" from 1300 to 1800 CE. woods hole oceanographic institute abrupt climate change